Circuit velocity step taper for suppression of backward wave oscillation in electron interaction devices

ABSTRACT

The output section of a helix type traveling wave tube or a transparent type traveling wave tube is provided with velocity step sections dimensioned so that a backward traveling circuit wave on the slow helix section synchronous with the slow space charge beam wave becomes synchronous with the fast space charge beam wave when traveling on the fast helix section. The step taper substantially suppresses backward oscillations which plague traveling wave tube devices. The phenomena provided by the structure of the invention is understood in terms of the coupled mode theory with particular attention to the Kompfner dip condition where the boundary conditions are such that the circuit voltages of the two half waves with different velocities add and the beam voltages cancel. All the RF energy then is transferred into the beam.

United States Patent Harper et al.

1451 Sept. 25, 1973 Inventors: Robert Harper, Concord; Jeffrey Wong, Boston; David Zavadil, Newton Center, all of Mass.

3,019,366 1/1962 Dunn 3l5/3.6

Primary ExaminerRudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Att0rneyHarold A. Murphy et al.

[57] ABSTRACT The output section of a helix type traveling wave tube [73] Assigneei Raytheml p y Lexington or a transparent type traveling wave tube is provided Mass with velocity step sections dimensioned so that a back- [22] Filed: JuIy 3 1972 ward traveling circuit wave on the slow helix section synchronous with the slow space charge beam wave be- PP N05 268,332 comes synchronous with the fast space charge beam wave when traveling on the fast helix section. The step 52 us. c1. 3l5/3.6 taper Substantially Suppresses backward Oscillations 51 int. Cl. H01j 25/34 which Plague traveling Wave tube devices- The P 581 Field of Search 315/35, 3.6, 39.3; homeha Provided by the Structure of the invention is 333/34 understood in terms of the coupled mode theory with particular attention to the Kompfner dip condition 56] References Cited where the boundary conditions are such that the circuit voltages of the two half waves with different velocities UNITED STATES PATENTS add and the beam voltages cancel. All the RF energy gzzetngl. then is transferred into the beam 2:762:948 9/1956 Field 3l5/3.6 2 Claims, 4 Drawing Figures -36 INPUT ATTENUATOR /6 2 SECTION, 20 @UTPUT DC /8 VA RI A B L E CONTROL /4 4g 62 4 iTTT/26 FAST SLOW VELOCITY VELOCITY /0 CIRCUIT CIRCUIT SECTION SECTION 44 62 46 PAIENTED 3,761,760

SHEET 10F 2 36 40 ;EE.%fi?0 OUT T 0c /& VARIABLE CONTROL FAST sLow VELOCITY T VELOCITY C|RCU|T CIRCUIT /0 1 sEcnoN SECTION 44 62 4s PouTPuT SECTION- w] 22 TPI 44 62 l R O F I L E 26 T PI I01 46 T /\I V 0 \TA .0 70 U E lo- 72 1 o 3 1 I Z 22 TPI I g 20 SECTION ALONE l I I 1 2 "30 I I I t Z AXIAL LENGTH (INCHES) TRAILING EDGE OF ATTENUATOR SECTION, 20

CIRCUIT VELOCITY sTEP TAPER FOR SUPPRESSION F BACKWARD WAVE OSCILLATION IN ELECTRON INTERACTION DEVICES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to traveling wave type electron interaction devices and more particularly to backward wave suppression in such devices.

2. Description of the Prior Art Electron interaction devices such as the traveling wave tube commonly incorporate an electromagnetic energy propagating circuit having a predetermined periodicity such as, for example, a helix for amplifying electromagnetic energy by extracting kinetic energy from an adjacent high power beam of electrons. A circuit wave desirably travels along the propagating structure at a velocity less than that of light to establish a synchronous interaction with the beam. Electric and magnetic fields of the electromagnetic energy on the circuit induce perturbations in the electron beam to form electron packets or bunches having a fundamental frequency component at about the frequency of the circuit wave. Numerous harmonic frequency components are also present in the bunched electron beam. Typically, in traveling wave devices the beam of electrons is translated along the axial length (2) of the propagating structure whose periodicity retards the velocity of the circuit waves until the synchronous relationship is established to optimize the net exchange of energy between the beam and the circuit waves.

Electron packets which traverse the interaction path at a group velocity considered to be in step with the retarded electromagnetic fields of the circuit wave move at substantially the same phase velocity as the circuit upon the establishment of the synchronous relationship. The electron beam, therefore, becomes simultaneously velocity and density modulated along the direction of travel until a saturation point is reached where the kinetic energy extracted is maximum and the packets become disarranged.

The synchronous relationship resulting in the interaction between electrons and the circuit wave fields produces high frequency amplification and/or oscillation which may be characterized as of the backward or forward wave type". In a backward wave device the electron beam travels at a velocity which is synchronous with the phase velocity of a traveling space harmonic component (minus 1) moving in a direction opposite to that of the energy transport along the propagating circuit. Such hannonics have negative-phase velocity while the group velocity is positive. Hence, the electron beam travels in one direction while the energy of the induced wave travels in the opposite direction. In forward wave devices thb electron beam and energy transport of the induced wave travel in the same direction. The propagating circuit in the backward wave type device is constructed so that the phase velocity of the fundamental frequency-determining component travels in a direction inverse to the direction of the electron group velocity. A forward wave device, however, is provided with a propagating circuit whose phase velocity characteristic of the fundamental frequency component is in the same direction as the electron group velocity. In the art the term fundamental" refers to the space harmonic component of an electromagnetic wave having the largest phase velocity.

It may also be noted from the prior art that the high frequency electric fields of the propagated circuit waves have transverse and axial components. The axial component accelerates and decelerates the velocity of the electrons and brings them into a favorable phase focussed position to thereby release their energy to the electric field of circuit wave and enhance its amplitude. The axial component, therefore, is rimarily responsible for the induced beam modulation. The efficiency parameter of the applicable devices is a measure of the energy converted from the kinetic energy on the beam to the RF energy in the wave traveling along the propagating circuit. This parameter is an arithmetic ratio of the RF energy output to the input power and generally efficiencies are limited by the high voltage supplies required to bias the propagating circuit and provides an electron beam source capable of transporting larger amounts of kinetic energy.

In the art numerous methods for increasing the efficiency have been advanced which provide for an optimizing of the positioning of the electrons within the electron packets to prolong the energy extraction. Such prior art techniques include changing the pitch of the helix structure in an appropriate manner to vary the phase velocity. Hence, a larger number of turns in the helix propagating circuit slows the phase velocity down while a fewer number of turns increases the phase velocity. An example of such a device which preconditions the electron beam modulation so as to deliberately shift the phase of the modulated electron bunches and introduce a desynchronization effect at low levels of beam modulation is found in U.S. Pat. No. 3,614,5l7, issued Oct. 19, 1971, to Norman J. Dionne, and assigned to the assignee of the present invention. An intermediate velocity is produced in the circuit wave at substantially low levels of beam modulation where the interaction between the electrons and the circuit waves is in the order of 0.1-l percent of a DC beam power. The preconditioning results in induced motion in the modulated beam bunches toward the forward portion of the deceleration field of the electron bunches. Efficiency enhancement in the range of 50 to 60 percent has resulted in the practice of such velocity shifting techniques.

Another example of prior art circuit wave perturbation is the so-called voltage jump technique described in U.S. Pat. No. 2,817,037, issued Dec. 17, I957, to R.W. Peter. This technique involves the application of a separate voltage potential jump to the propagating structure, particularly, at intermittent points where the electrons in the beam have a tendency to slow down. The technique requires the addition of numerous voltage supplies with accompanying increased expense.

In all of the foregoing efficiency enhancement techniques, a problem remains in that kinetic energy in the beam may be coupled to the minus I space harmonic waves to result in the backward wave phenomena inherent in such periodic type energy propagating structure which has no parallel in smooth waveguide propagating structures. The invention, therefore, is directed to the problem of controlling coupling to the minus 1 space harmonics in forward traveling wave devices to suppress backward wave oscillations. The fast and slow space charge beam waves which are the normal modes of excitation possible on a constant DC velocity electron beam are included as the means for accomplishing this result.

SUMMARY OF THE INVENTION In accordance with the invention means for suppression of backward wave oscillations comprise a velocity step taper section in an electromagnetic energy propagating circuit near the output end where the minus 1 space harmonic of the circuit typically couples strongly with the slow space charge wave on the electron beam. In the embodiments having an internal attenuator section the fast velocity circuit section is disposed closer to the trailing edge of the attenuator relative to the output coupling means. In the case of the transparent type traveling wave device which does not provide for any gain the velocity step is disposed nearer to the input end. The dimensions of the fast and slow circuit velocity sections are selected to provide for the coupling of the backward wave circuit components to the fast space charge beam wave in the fast circuit section. The length of the slow velocity circuit section is greater than that necessary for the efficient commencement of backward wave oscillation when the fast velocity circuit section is reached these backward waves couple to the fast space charge beam wave. In accordance with the coupled mode theory in the Kompfner dip condition substantially all of the backward wave RF energy goes into the beam similar to a directional coupler and there is no exponential gain.

In an exemplary embodiment of the invention a high gain traveling wave tube having a helix circuit and an approximate 6.5 kv beam voltage, a 26 T.P.I. slow velocity circuit section has a natural BWO oscillation frequency of approximately 18.55 GHz. The remaining fast velocity circuit section with a pitch of 22 T.P.I. commencing approximately 1.0 inches from the trailing edge of the attenuator section has a natural BWO frequency of 17.14 GHz. The length of the fast helix section adjacent the trailing edge of the attenuator is shorter than that required to support backward wave oscillations. The slow velocity circuit section on the other hand has a greater length than that necessary for the commencement of backward wave oscillation. The intermediate taper section has an overall length of 0.250 inches. The result of the energy transfer, therefore, of the minus 1 space harmonic components to the fast space charge beam wave results in substantial attenuation of such backward wave energy.

The dimensions required for the implementation of the invention are readily calculated utilizing the advances in the art with respect to computerized model simulation of electron interaction device operation. This computerized model for setting the optimum conditions of the propagating circuit parameters was achieved by such authorities as J.E. Rowe, P.I(. Tien, H.C. Poulter, M.E. El-Shandwili and AJ. Giarola. References to such art include: Transactions, Professinal Group of Electron Devices, IRE, Vol. ED-3, Jan. 1956, pps. 39-57; Technical Report No. 73, ONR Contract NGonr25 l (07), Electronics Research Laboratory, Stanford University, Jan. 1954; Bell System Technical Journal, Vol. 35, March 1956, pps. 349-374; and University of Michigan Technical Report No. 85, Electron Physics Laboratory, Ann Arbor, Michigan, June 1965. The details of the referenced computerized programs along with the mathematical computations have been enumerated in the aforereferenced US. Pat. No.

3,614,517 and have not been repeated in the present specification.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be readily understood after consideration of the following description of a preferred embodiment and reference to the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a traveling wave electron interaction device embodying the invention;

FIG. 2 is a plot of the amplitude of the circuit wave profile on the fast velocity circuit section of the embodiment of the invention;

FIG. 3 is a plot of the amplitude of the circuit wave profile on the slow velocity circuit section of the illustrative embodiment of the invention; and

FIG. 4 is a Brillouin diagram of the output helix section in the backward wave interaction region.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 an exemplary embodiment of the traveling wave tube 10 for amplification of electromagnetic energy shown comprises a circuit wave guiding means such as a helix 12 extending along the longitudinal axis of a hermetically sealed envelope 14. The electromagnetic energy to be amplified is coupled to the input section of helix 12 by conductor means 16 and output coupling means 18 are disposed at the opposing end. An attenuator section 20 is intermediately disposed as is conventional in traveling wave devices with high gain capability in the 50-60 db region to prevent feedback of energy from the output end. In the transparent-type traveling wave tube devices which also can utilize the structure of the invention no internal attenuator is necessary and, therefore, the overall length is considerably shorter. Such transparent-type traveling wave devices are typically employed as power boosters and are coupled with traveling wave tube amplifiers to provide additional gain in the range of 10 db over exceedingly wide frequency ranges of about an octave.

An electron beam source 22 is disposed adjacent the input end 16 and is of the directly-heated gun type including means for directing a stream of electrons 24 along the axis of the device to interact with energy on the helix 12. A collector electrode 26 at the opposing end of the envelope provides for collection of spent electrons after traversing the interaction path. Electron source 22 comprises an emissive cathode 28 provided with a curvature to assist in the focussing of the electron beam together with a heater coil 30. Leads 32 provide for the connection of the cathode structure to appropriate DC voltage supplies and are hermetically sealed in the envelope walls. A grid control electrode 34 may be disposed adjacent to the cathode emitter and be biased by a suitable DC variable supply 36. An accelerator electrode 38 provides for the beam focus sing and is conventionally biased positively by a suitable DC voltage supply to accelerate the electrons along the desired trajectory path. Magnetic field producing means 40 surround the tube envelope 14 and provide for a longitudinal magnetic field parallel to the axis of the device to confine the electrons to the desired beam path.

After attenuator section 20 an output helix section 42 continues the energy propagating circuit to the output end 18. It is to this output helix section that attention is now directed and in accordance with the invention two different velocity circuit sections 44 and 46 are provided with an intermediate taper section 62 to substantially suppress backward wave interaction. Section 44 is referred to as the fast velocity circuit section and has a fewer number of turns, for example, 22 T.P.I., than the remainder of the output section. The velocity of the circuit waves in this section, therefore, are faster. Helix section 46 which is referred to as the slow velocity circuit section has a tighter or closer spaced number of helix turns, illustratively 26 T.P.I.

Referring now to FIGS. 2, 3 and 4 the circuit wave profiles as well as Brillouin diagram for the applicable output helix section will be discussed. The coupledmode theory states that there are two circuit waves, a forward and a backward wave, as well as two space charge beam waves, a slow and a fast one, which may be coupled together depending on their relative velocities. The circuit wave carries positive power and the slow space charge beam wave negative power so that oscillatory electron bunching arises when there is a synchronous relationship with a net exchange of energy leading to growing and attenuating waves on the propagating circuit. When the circuit wave and fast space charge beam wave couple new waves arise with no attenuation and behavior is like that of a directional coupler with the energy flowing alternately back and forth between the beam and the circuit without any exponential gain. The boundary condition is such that when a fast space charge wave is at circuit velocity the new circuit wave voltages add and the beam voltages cancel. Hence, all the RF power goes into the beam. This condition is referred to in the art as the Kompfner dip condition. In FIG. 4 a Brillouin diagram is shown for the region of the propagating circuit near the output end where the backward wave oscillation phenomena exists. The diagram plots the beam velocity indicated by line 48 together with the fast space charge beam wave indicated by dotted line 50 and the slow space charge beam wave indicated by the dotted line 52. Next, the minus 1 space harmonic for the 22 T.P.I. circuit section as well as the 26 T.P.I. section are plotted as indicated by the solid lines 54 and 56. The plus 1 space harmonic for the respective circuit sections is indicated by solid lines 58 and 60, respectively.

In accordance with the aforereferenced computerized modeling technique the natural oscillation frequencies of the circuit output sections 44 and 46 at 6.15 kilovolt beam voltage and 0.700 amperes beam current were calculated as follows:

TABLE I Start Start Osc. Osc. BWO Current Length Freq. T.P.I. Amps. Inches 61-12 22 0.700 1.40 17.14 26 0.700 1.23 18.56

The interaction point at which the respective beam and circuit waves intersect with the minus 1 space harmonic for the respective circuit sections have been indicated in FIG. 4. These plotted calculations will permit the setting of the optimum conditions to cause the coupling of the backward wave oscillation components to the fast space charge beam wave. As is well known in the art, if only a slow wave is on the beam the total power carried by the beam is less than the DC power.

Conversely, if only a fast wave is on the beam, the total power is greater than the DC beam power. The relative velocitiesof the slow and fast waves are obtained from the following equations:

where U, DC velocity of electrons and cuq reduced plasma frequency.

The output helix section 42 commences with a fast velocity section 44 having illustratively 22 T.P.I. The length of this section is shorter than the required oscillation length for the coupling of the backward wave oscillation components at a frequency of 17.14 GHz. This overal oscillation length was calculated to be 1.40 inches and, therefore, an overall length of approximately one inch from the trailing edge of the attenuator section 20 meets this condition. A step down taper section 62 forms the transition structure from the fast velocity circuit section 44 to the tighter pitch section 46 having illustratively 26 T.P.I. which forms the slow velocity circuit. The overall length of the taper in the illustrative embodiment was determined to be 0.250 inches in length. The height of the velocity step down taper 62 is adjusted so that a backward traveling circuit wave on the slow helix section 46 which is synchronous with the slow space charge beam wave is synchronous with the fast space charge beam wave when traveling on the fast helix circuit section 44. The adjustment of the number of turns in the taper section is also calculated by the computerized techniques. The slow velocity circuit section 46 has a total overall length of about 2.00 inches which it will be noted is in excess of the required length of 1.23 inches to couple the backward wave oscillations at a natural frequency of 18.56 GI-Iz.

It will be noted that a minus 1 space harmonic traveling along the 26 TR]. circuit section 46 intersects the slow space charge beam wave at approximately 18.70 61-12; is synchronous with the beam velocity at approximately 19.30 GI-Iz; and intersects the fast space charge beam wave at 19.90 GHz. In accordance with the coupled-mode theory the backward circuit traveling wave components indicated by the arrow 64 traversing the 26 T.P.I. circuit section travel over a length greater than that necessary for the start of the backward wave oscillations. In this section then there is relatively little backward wave gain and the backward wave circuit component tends to propagate toward the electron source means end of the deivce in synchronous relationship with the slow space charge beam wave which has the closest frequency of 18.70 GI-Iz.

After traversing the taper section 62 the fast velocity circuit section 44 is encountered by this backward traveling circuit wave and, as previously noted, the natural backward wave oscillations frequency in this section is 17.14 GI-Iz. As a result, no backward wave oscillations can occur at this frequency since it is too far removed from the beam velocity or slow space charge beam wave velocity. The frequency of approximately 18.70 GI-Iz, however, is relatively close to the fast space charge beam wave velocity indicated by dashed line 50 and the point of intersection is at 18.38 GHz. As a result, there is synchronous coupling between this backward circuit traveling wave and the fast space charge beam wave to create an attenuating circuit wave moving in the direction of the electron beam end. The effect of the different velocity sections is shown by the line 66 and arrows indicating a shift of a synchronous relationship with a slow space charge beam wave to the fast space charge beam wave. In accordance with the previously enumerated coupled-mode theory the coupling to the fast space charge beam wave results in behavior like a directional coupler with the energy alternating back and forth between the beam and circuit without exponential gain. The circuit voltages add and the beam voltages cancel and substantially all of the backward wave component energy goes directly into the beam. A new method of effectively suppressing backward wave oscillations in a forward wave traveling wave tube device has, therefore, evolved.

Referring now to FIG. 2 the results of the provision of the fast and slow velocity circuit output helix sections is clearly indicated in a circuit profile diagram plotted with axial length and small signal gain in db coordinates. The 22 T.P.I. section 44 without any change in the number of turns has a backward wave oscillation condition indicated by the dashed line 68. This oscillation condition has a calculated length of 1.40 inches, however, as previously noted, the section now has a shorter length than that required for the backward wave oscillations and, hence, no efficient backward wave oscillations at the natural resonant frequency range of about 17.14 Gl-lz arise in this section. The resultant attenuating backward circuit wave is indicated by the solid line 70 with the optimum oscillation conditions indicated by the dip 72.

Referring next to FIG. 3 the circuit profile diagram is plotted for the slow velocity circuit section 46 having, illustratively, 26 T.P.l. The calculated length for the commencement of backward wave oscillations in this section is substantially greater than that required for initiation of oscillations. Hence, where the optimum synchronous conditions for such a section alone are indicated by the dashed lines 74 the actual attenuator circuit wave which arises is indicated by the solid line 76 with values as high as plus 10 db. The net result of the overall structure is a poorly formed attenuating circuit wave traveling toward the electron beam source end. When this circuit wave reaches the fast velocity circuit section synchronous coupling occurs with the fast space charge wave with a substantial transfer of energy from the circuit to the beam.

The structure of the invention, therefore, effectively suppresses backward wave oscillation components by coupling the energy from the minus 1 circuit wave space harmonic to the fast space charge beam wave. In addition to the implementation of the invention by the varying of the number of turns in the respective fast and slow velocity circuit sections, as well as taper section, other means exist in the art such as, for example, adjustments of the diameter of the helix in the respective sections. Other variations, modifications and alterations will be evident to those skilled in the art. It is intended, therefore, that the foregoing description of an illustrative embodiment of the invention be considered broadly and not in a limiting sense.

We claim: 1. A traveling wave electron interaction device comprising:

circuit wave guiding structure for propagating electromagnetic energy having input and output coupling means; means for generating and directing an electron beam having slow and fast space charge wave components along a path adjacent to said circuit guiding structure to interact in an energy exchanging relationship with the propagating circuit waves; and means for substantially suppressing backward traveling circuit wave components having a predetermined natural oscillation frequency initiation determining length characteristic; said suppressing means comprising a circuit guiding structure having a slow and a fast velocity circuit section; said slow velocity circuit section being disposed closer to said output coupling means and having a length greater than said natural backward wave oscillation frequency length to result in a synchronous relationship of said backward wave components with said slow space charge beam wave; said fast velocity circuit section having a length shorter than said natural backward wave oscillation frequency length to result in a synchronous relationship of said backward wave components with said fast space charge beam wave; said slow and fast circuit sections being separated by an intermediate circuit section having a gradually varying velocity characteristic. 2. The device according to claim 1 wherein said circuit wave guiding structure input coupling means is separated from said suppressing means by an attenuator section. 

1. A traveling wave electron interaction device comprising: circuit wave guiding structure for propagating electromagnetic energy having input and output coupling means; means for generating and directing an electron beam having slow and fast space charge wave components along a path adjacent to said circuit guiding structure to interact in an energy exchanging relationship with the propagating circuit waves; and means for substantially suppressing backward traveling circuit wave components having a predetermined natural oscillation frequency initiation determining length characteristic; said suppressing means comprising a circuit guiding structure having a slow and a fast velocity circuit section; said slow velocity circuit section being disposed closer to said output coupling means and having a length greater than said natural backward wave oscillation frequency length to result in a synchronous relationship of said backward wave components with said slow space charge beam wave; said fast velocity circuit section having a length shorter than said natural backward wave oscillation frequency length to result in a synchronous relationship of said backward wave components with said fast space charge beam wave; said slow and fast circuit sections being separated by an intermediate circuit section having a gradually varying velocity characteristic.
 2. The device according to claim 1 wherein said circuit wave guiding structure input coupling means is separated from said suppressing means by an attenuator section. 